ROME-- The spiritual leader of 1.2 billion Catholics, Pope Benedict XVI, surprised the world Monday by saying will resign at the end of the month "because of advanced age."
It's the first time a pope has resigned in nearly 600 years.
"Strength of mind and body are necessary, strength which in the last few months has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me," the pope said, according to the Vatican.
After Benedict's resignation becomes effective on February 28, cardinals will meet to choose a new leader for the church.
"Before Easter, we will have the new pope," the Rev. Federico Lombardi, a Vatican spokesman, said at a news conference.
The decision was not impulsive, he said.
"It's not a decision he has just improvised," Lombardi said. "It's a decision he has pondered over."
After his resignation, Benedict, 85, will probably retire to a monastery and devote himself to a life of reflection and prayer, he said.
Archbishop Vincent Nichols, the president of the Catholic Bishops' Conference of England and Wales, said the decision "shocked and surprised everyone."
"Yet, on reflection, I am sure that many will recognise it to be a decision of great courage and characteristic clarity of mind and action," he said.
Local scholars are also weighing-in.
"I think what he's done, more than anything, is acknowledge that being Pope is very difficult and strenuous job," said Justin Catanoso, Director of Journalism at Wake Forest University.
Catanoso wrote about his cousin, Saint Gaetano Catanoso, who was canonized by Pope Benedict in the book "My Cousin the Saint."
"This is a voluntary retirement, this is holiest of the holy saying I`m no longer capable of doing this job and rather that wither and die in front of the world, very much like Pope John Paul the second did. Benedict said I'm going to do it differently."
"This is an 85-year-old man that under normal circumstances no one would question an 85-year-old man deciding to retire," says Dr. David Yamane a Catholic Scholar and Professor of Religion at Wake Forest University.
"It's so unprecedented it makes you wonder is something else going on that hasn't been reveled? On the positive side people who are looking for a more modern Catholic church this could be a step toward that."
Benedict -- born Joseph Ratzinger -- will not be involved in choosing a new pope or in guiding the church after his resignation.
Benedict was elected pope in 2005 after the death of Pope John Paul II, the third-longest-serving leader of the Catholic Church.
He has served during a time in which the church is declining in his native Europe but expanding in Africa and Latin America.
His papacy also has been marked with a series of scandals and controversies, including hundreds of new allegations of sexual abuse by priests.
Ratzinger was born on April 16, 1927, in Marktl Am Inn, Bavaria, a heavily Catholic region of Germany.
He spent his adolescent years in Traunstein, near the Austrian border, during the Nazi regime of Adolf Hitler.
Ratzinger wrote in his memoirs that school officials enrolled him in the Hitler Youth movement against his will when in 1941, when he was 14.
He said he was allowed to leave the organization because he was studying for the priesthood, but was drafted into the army in 1943. He served with an anti-aircraft unit until he deserted in the waning days of WW II.
After the war, he resumed his theological studies and was ordained in 1951. He received his doctorate in theology two years later and taught dogma and theology at German universities for several years.
In 1962, he served as a consultant during the pivotal Vatican II council to Cardinal Frings, a reformer who was the archbishop of Cologne, Germany.
As a young priest, Ratzinger was on the progressive side of theological debates, but began to shift right after the student revolutions of 1968, CNN Vatican analyst John Allen Jr. said.
In his book "Cardinal Ratzinger: The Vatican's Enforcer of the Faith," Allen says Ratzinger is a shy and gentle person whose former students spoke of him as a well-prepared and caring professor.
Pope Paul VI named him archbishop of Munich in 1977 and promoted him to cardinal the next month. Ratzinger served as archbishop of Munich until 1981, when he was nominated by John Paul II to be the head of the Congregation for the Doctrine of the Faith, a position he held until his election as pope.
He became dean of the College of Cardinals in November 2002 and in that role called the cardinals to Rome for the conclave that elected him the 265th pope.
In his initial appearance as pope, he told the crowd in St. Peter's Square that he would serve as "a simple and humble worker in the vineyards of the Lord."
He is the sixth German to serve as pope and the first since the 11th century.
The last pope to resign was Gregory XII in 1415. He did so to end a civil war within the church in which more than one man claimed to be pope.
Full text of Pope Benedict XVI's declaration:
"I have convoked you to this Consistory, not only for the three canonizations, but also to communicate to you a decision of great importance for the life of the Church. After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry. I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering. However, in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me. For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.
"Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects. And now, let us entrust the Holy Church to the care of Our Supreme Pastor, Our Lord Jesus Christ, and implore his holy Mother Mary, so that she may assist the Cardinal Fathers with her maternal solicitude, in electing a new Supreme Pontiff. With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer.
"From the Vatican, 10 February 2013
"BENEDICTUS PP XVI"
CNN contributed to this report